Microwave limiter comprising a thin conductor in the transmission means located in proximity to ferrite material



3,356,967 N THE Dec. 5, 1967 w. M. HONIG Y MICROWAVE LIMITER COMPRISING A THIN CONDUCTOR I TRANSMISSION MEANS LOCATED IN PROXIMlTY TO FERRITE MATERIAL Filed April 15, 1965 FIG. 2

DIELECTRIC THERMALLY CONDUCTIVE FERRITE POWER OUT POWER IN FIG. 4

FIG. 5

INVENTOR. WILLIAM M. HONIG ATTORNEYS United States Patent 3,356,967 MICRQWAVE LIMITER COMPRISING A THIN CONDUCTOR IN THE TRANSMIS- SION MEANS LOCATED IN PROXIMITY TO FERRITE MATERIAL William M. Honig, 6801 Bay Parkway, Brooklyn, NY. 11204 Filed Apr. 15, 1965, Ser. No. 448,499 4 Claims. (Cl. 333--17) ABSTRACT OF THE DISCLOSURE A wide band ferrite microwave limiter comprises a coaxial cable having its center conductor interrupted and a thin Wire of the order of 35 millionths of an inch in diameter breaching the gap. The thin wire is bonded to a thermally conductive dielectric material such as beryllium oxide and a ferrite material is placed adjacent thereto. A pair of magnets extend through the external conductor of the coaxial cable to provide a field for varying the limiter power level.

This invention relates to microwave limiters, and, in particular, to a wide band ferrite microwave limiter in which the power output may be set to different levels.

As its name implies, a microwave limiter is a device which limits microwave power to a substantially fixed level, i.e. power output remains substantially constant regardless of the increase in input power. Such devices have utility in the protection of the crystal mixers of micro- Wave receivers and also in certain specialized microwave signal sensing systems. The present invention would have utility in all situations where it is desired to limit microwave power to a predetermined level.

A drawback of the prior art microwave limiters is the narrow frequency range over which they are operable, and a principal object of the present invention is to provide microwave limiter which is operable over a relatively wide frequency range.

Another object of the invention is to provide a microwave limiter in which the power output may readily be set to any one of a wide range of different levels.

Yet another object of the invention is to provide a solid state microwave limiter incorporating all of the attendant advantages such as ruggedness, compactness, and reliability.

The manner in which the above and other objects of the invention are accomplished is more fully described below with reference to the attached drawings, wherein:

FIGURE 1 is a sectional side view of a microwave limiter in accordance with the invention;

FIGURE 2 is a cross sectional view along the line 22 of FIGURE 1;

FIGURE 3 is a graph showing the power output as a function of the input power for explanatory purposes;

FIGURE 4 is a side sectional view of another embodiment of the invention; and

FIGURE 5 is a side sectional view of still another embodiment of the invention.

Referring to FIGURES 1 and 2, a conventional coaxial transmission line is shown comprising a cylindrical outer conductor and a center conductor 13 concentric therewith. Outer conductor 10 includes suitable slots (not numbered) in which permanent magnets 14 and 16, oppositely poled, are situated as shown. The invention may be modified for use with waveguide or strip transmission line or other form of transmission line as would be obvious to those skilled in the art.

The center conductor 12 is interrupted in the portion adjacent the poles 14 and 16, and in this area a thin Wire 18 is axially situated. This thin wire 18 is bonded to a 3,356,967 Patented Dec. 5, 1967 thermally conductive dielectric block 22, which, for example, may consist of beryllium oxide or titanium oxide. A ferrite slab 24, consisting of Poly or Single-Crystal Yttrium Iron Garnet, for example, is mounted with its upper edge closely adjacent or touching thin wire 18. Thermal block 22 and ferrite slab 24 may be mounted in any conventional manner. Standard Wollaston wire techniques may be used for this purpose whereby a gold wire, sheathed in silver, is cemented (or otherwise bonded) to the dielectric 22 (or ferrite 24) and the silver sheathing thereafter etched away. The thin wire 18 may be a small diameter gold wire having a diameter between 0.100 and 0.005 mil, and a length of approximately mils.

The operation of the device is as follows. Assume that the RF input power is arriving in the direction of arrow 26. At low power levels, thin wire 18 does not affect the operation of the coaxial cable and the power output rises linearly with the input power. This is shown on the portion of the graph (FIGURE 3) indicated at 28.

As the power input continues to increase, the RF magnetic field produced at the thin wire 18 also increases until it reaches a critical level shown at 30 in FIG. 3 at which point spin waves are launched into the ferrite slab 24. The phenomenon of spin waves is known in the art and an explanation thereof may be found in Microwave Ferrites and Ferrimagnetics by Lax & Button and published by McGraw-Hill. For the present purposes it is suflicient to note that when this critical level is reached, the launching of the spin waves into the ferrite produces a change in the RF losses and permeability of the ferrite, which, in turn, changes the impedance of the microwave structure. This impedance change is such that further input power is absorbed or reflected, so that the power output remains substantially constant as shown by line 32 in FIG. 3.

The shape of the various components is not critical (except that wire 18 preferably has at least one thin dimension). The losses and permeability of the ferrite 24 is a function of the RF magnetic field in that region and varies inversely with the diameter of the thin wire 18. Hence, the thinner the wire the better the limiting action, but the thinner the wire the greater the temperature rise it would normally experience and for this reason the wire is provided with a heat sink in either the beryllium oxide 22 (or possibly alternatively or additionally in the ferrite 24). This thermal bond is important since heat produced by the wire 18 must be conducted away lest the wire melt during operation. If desired, a heat conductive dielectric fiuid (e.g. silicone oil) may be used in place of the block 22 or may be used to provide thermal contact between the wire 18 and block 22.

The device operates over a large frequency range and, for example, would have utility from approximately 50 to 14,000 megacycles. The DC magnetic field produced by magnets 14 and 16 determines the frequency range over which the device is operable, and at each frequency region a different magnitude of DC magnetic field is necessary to cause the ferrite to behave as explained above. It is therefore desirable to produce a non-uniform magnetic field so that full frequency range may be covered, and for this purpose part of the center conductor 12 may be made of magnetic material in order to provide greater nonlinearity of the DC magnetic field near the region of the wire 18 and ferrite 24. This will also permit smaller field magnets 14 and 16 to be used. The constant magnetic field may be 3,000 to 4,000 gauss where it is at its highest value. The preferred construction of the apparatus described also will provide a high radio frequency magnetic field which is on the order of 20 oersteds (in air).

As noted above, the wire 18 will customarily be thermally bonded to the beryllium oxide 22. It is then a simple matter to vary the position of the ferrite or to vary the vertical positioning of the thin wire (and its proximity to ferrite 24) by conventional means (not shown) to alter the operating level of the limiter (that is the microwave power necessary to generate the critical RF magnetic field). This provides a convenient manner of setting the limiter to different power levels as indicated by the respective breaking points 30, 34 and 36 in FIGURE 3.

The magnetic field in the structure may be axial, radial, or circumferential and various coupling, configurations may be used. For example, as schematically shown, FIG- URE 4 illustrates a parallel input-output structure employing loop coupling in which thin wires 18a and 18b cooperate with the ferrite slab 24 as explained above to limit the power transfer. FIGURES schematically shows a perpendicular input output structure employing thin wires 18c and 18d for coupling purposes as well asfor the limiter action.

Although preferred embodiments of the invention have been shown and described the invention is not so limited and should be defined by reference to the following claims.

What is claimed is:

, 1. A broad band microwave limiter, comprising radio frequency transmission means, a ferrite material in said transmission means, a thin conductor in said transmission means located in proximity to said ferrite material, the diameter of said thin conductor being between 5 and 100 millionths of an inch, means for producing a magnetic field in said ferrite, said thin conductor being located with respect to said ferrite material such that when the RE power transmitted through said transmission means ex- .ceeds a pre-determined limit, the magnetic field produced by said thin conductor launches spin waves in said ferrite material thereby limiting the transfer of said power to a pre-determined level, and thermal means for conducting the heat generated in said thin conductor therefrom.

2. A microwave limiter, according to claim 1, wherein said thermal means comprises a thermally conductive dielectric material, said thin conductor being bonded in a thermally conductive relation to said thermally conductive material.

3. A microwave limiter according to claim 2, wherein said transmission means comprises a coaxial cable having a center conductor, with said thin conductor serving as a portion of said center conductor.

4. A microwave limiter according to claim 3, further including means for creating a non-uniform magnetic field in said ferrite material.

References Cited UNITED STATES PATENTS 3,082,383 3/1963 Stern 33324.1 3,221,395 12/1965 Nielsen 33324.2 3,246,262 4/1966 Wichert 33324.1

ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

; P. L. GENSLER, Assistant Examiner. 

1. A BROAD BAND MICROWAVE LIMITER, COMPRISING RADIO FREQUENCY TRANSMISSION MEANS, A FERRITE MATERIAL IN SAID TRANSMISSION MEANS, A THIN CONDUCTOR IN SAID TRANSMISSION MEANS LOCATED IN PROXIMITY TO SAID FERRITE MATERIAL, THE DIAMETER OF SAID THIN CONDUCTOR BEING BETWEEN 5 AND 100 MILLIONTHS OF AN INCH, MEANS FOR PRODUCING A MAGNETIC FIELD IN SAID FERRITE, SAID THIN CONDUCTOR BEING LOCATED WITH RESPECT TO SAID FERRITE MATERIAL SUCH THAT WHEN THE RF POWER TRANSMITTED THROUGH SAID TRANSMISSION MEANS EXCEEDS A PRE-DETERMINED LIMIT, THE MAGNETIC FIELD PRODUCED BY SAID THIN CONDUCTOR LAUNCHES SPIN WAVES IN SAID FERRITE MATERIAL THEREBY LIMITING THE TRANSFER OF SAID POWER TO A PRE-DETERMINED LEVEL, AND THERMAL MEANS FOR CONDUCTING THE HEAT GENERATED IN SAID THIN CONDUCTOR THEREFROM. 